Bladder snail. Often found in ponds, hence often called pond snails, but there are at least a couple of other common pond snail species too. Bladder snails rarely exceed 3/8 inch in length, are quite fast movers compared to most other snails, and reproduce very quickly. They lay eggs in small blobs of clear jelly like material, which hatch out upward of a dozen babies at a time.
If you have shrimp, try crushing any bladder snails you find and letting them sink, as shrimp will eat the snail bodies with great relish.